The second, third, and fourth extended file systems, or ext2, ext3, and
ext4 as they are commonly known, are Linux file systems that have his-
torically been the default file system for many Linux distributions.
They are general purpose file systems that have been designed for
extensibility and backwards compatibility. In particular, file systems
previously intended for use with the ext2 and ext3 file systems can be
mounted using the ext4 file system driver, and indeed in many modern
Linux distributions, the ext4 file system driver has been configured
handle mount requests for ext2 and ext3 file systems.
FILE SYSTEM FEATURES
A file system formated for ext2, ext3, or ext4 can be have some collec-
tion of the follow file system feature flags enabled. Some of these
features are not supported by all implementations of the ext2, ext3,
and ext4 file system drivers, depending on Linux kernel version in use.
On other operating systems, such as the GNU/HURD or FreeBSD, only a
very restrictive set of file system features may be supported in their
implementations of ext2.
Enables the file system to be larger than 2^32
blocks. This feature is set automatically, as
needed, but it can be useful to specify this feature
explicitly if the file system might need to be
resized larger than 2^32 blocks, even if it was
smaller than that threshold when it was originally
created. Note that some older kernels and older
versions of e2fsprogs will not support file systems
with this ext4 feature enabled.
This ext4 feature enables clustered block alloca-
tion, so that the unit of allocation is a power of
two number of blocks. That is, each bit in the what
had traditionally been known as the block allocation
bitmap now indicates whether a cluster is in use or
not, where a cluster is by default composed of 16
blocks. This feature can decrease the time spent on
doing block allocation and brings smaller fragmenta-
tion, especially for large files. The size can be
specified using the -C option.
Warning: The bigalloc feature is still under devel-
opment, and may not be fully supported with your
kernel or may have various bugs. Please see the web
for details. May clash with delayed allocation (see
This feature requires that the extent features be
block numbers for a particular inode to physical
blocks on the storage device to be stored using an
extent tree, which is a more efficient data struc-
ture than the traditional indirect block scheme used
by the ext2 and ext3 file systems. The use of the
extent tree decreases metadata block overhead,
improves file system performance, and decreases the
needed to run e2fsck(8) on the file system. (Note:
both extent and extents are accepted as valid names
for this feature for historical/backwards compati-
This ext4 feature reserves a specific amount of
space in each inode for extended metadata such as
nanosecond timestamps and file creation time, even
if the current kernel does not current need to
reserve this much space. Without this feature, the
kernel will reserve the amount of space for features
currently it currently needs, and the rest may be
consumed by extended attributes.
For this feature to be useful the inode size must be
256 bytes in size or larger.
This feature enables the use of extended attributes.
This feature is supported by ext2, ext3, and ext4.
This feature enables the storage file type informa-
tion in directory entries. This feature is sup-
ported by ext2, ext3, and ext4.
This ext4 feature allows the per-block group meta-
data (allocation bitmaps and inode tables) to be
placed anywhere on the storage media. In addition,
mke2fs will place the per-block group metadata
together starting at the first block group of each
"flex_bg group". The size of the flex_bg group can
be specified using the -G option.
Create a journal to ensure filesystem consistency
even across unclean shutdowns. Setting the filesys-
tem feature is equivalent to using the -j option.
This feature is supported by ext3 and ext4, and
ignored by the ext2 file system driver.
This ext4 feature allows files to be larger than 2
terabytes in size.
kernels when a file larger than 2 gigabytes is cre-
ated. Very old kernels could not handle large
files, so this feature flag was used to prohibit
those kernels from mounting file systems that they
could not understand.
This feature indicates that there will only at most
two backup superblock and block group descriptors.
The block groups used to store the backup superblock
and blockgroup descriptors are stored in the
superblock, but typically, one will be located at
the beginning of block group #1, and one in the last
block group in the file system. This is feature is
essentially a more extreme version of sparse_super
and is designed to allow the a much larger percent-
age of the disk to have contiguous blocks available
for data files.
This ext4 feature allows file systems to be resized
on-line without explicitly needing to reserve space
for growth in the size of the block group descrip-
tors. This scheme is also used to resize file sys-
tems which are larger than 2^32 blocks. It is not
recommended that this feature be set when a file
system is created, since this alternate method of
storing the block group descriptor will slow down
the time needed to mount the file system, and newer
kernels can automatically set this feature as neces-
sary when doing an online resize and no more
reserved space is available in the resize inode.
This ext4 feature provides multiple mount protection
(MMP). MMP helps to protect the filesystem from
being multiply mounted and is useful in shared stor-
Create quota inodes (inode #3 for userquota and
inode #4 for group quota) and set them in the
superblock. With this feature, the quotas will be
enabled automatically when the filesystem is
Causes the quota files (i.e., user.quota and
group.quota which existed in the older quota design)
to be hidden inodes.
This file system feature indicates that space has
been reserved so the block group descriptor table
can be extended by the file system is resized while
ext3, and ext4 file system. It indicates that
backup copies of the superblock and block group
descriptors be present only on a few block groups,
and not all of them.
This ext4 file system feature indicates that the
block group descriptors will be protected using
checksums, making it safe for mke2fs(8) to create a
file system without initializing all of the block
groups. The kernel will keep a high watermark of
unused inodes, and initialize inode tables and block
lazily. This feature speeds up the time to check
the file system using e2fsck(8), and it also speeds
up the time required for mke2fs(8) to create the
This section describes mount options which are specific to ext2, ext3,
and ext4. Other generic mount options may be used as well; see
mount(8) for details.
Mount options for ext2
The `ext2' filesystem is the standard Linux filesystem. Since Linux
2.5.46, for most mount options the default is determined by the
filesystem superblock. Set them with tune2fs(8).
Support POSIX Access Control Lists (or not).
Set the behavior for the statfs system call. The minixdf behav-
ior is to return in the f_blocks field the total number of
blocks of the filesystem, while the bsddf behavior (which is the
default) is to subtract the overhead blocks used by the ext2
filesystem and not available for file storage. Thus
% mount /k -o minixdf; df /k; umount /k
Filesystem 1024-blocks Used Available Capacity Mounted on
/dev/sda6 2630655 86954 2412169 3% /k
% mount /k -o bsddf; df /k; umount /k
Filesystem 1024-blocks Used Available Capacity Mounted on
/dev/sda6 2543714 13 2412169 0% /k
(Note that this example shows that one can add command line
options to the options given in /etc/fstab.)
check=none or nocheck
No checking is done at mount time. This is the default. This is
fast. It is wise to invoke e2fsck(8) every now and then, e.g.
at boot time. The non-default behavior is unsupported
grpid|bsdgroups and nogrpid|sysvgroups
These options define what group id a newly created file gets.
When grpid is set, it takes the group id of the directory in
which it is created; otherwise (the default) it takes the fsgid
of the current process, unless the directory has the setgid bit
set, in which case it takes the gid from the parent directory,
and also gets the setgid bit set if it is a directory itself.
The usrquota (same as quota) mount option enables user quota
support on the filesystem. grpquota enables group quotas sup-
port. You need the quota utilities to actually enable and manage
the quota system.
Disables 32-bit UIDs and GIDs. This is for interoperability
with older kernels which only store and expect 16-bit values.
oldalloc or orlov
Use old allocator or Orlov allocator for new inodes. Orlov is
resgid=n and resuid=n
The ext2 filesystem reserves a certain percentage of the avail-
able space (by default 5%, see mke2fs(8) and tune2fs(8)). These
options determine who can use the reserved blocks. (Roughly:
whoever has the specified uid, or belongs to the specified
sb=n Instead of block 1, use block n as superblock. This could be
useful when the filesystem has been damaged. (Earlier, copies
of the superblock would be made every 8192 blocks: in block 1,
8193, 16385, ... (and one got thousands of copies on a big
filesystem). Since version 1.08, mke2fs has a -s (sparse
superblock) option to reduce the number of backup superblocks,
and since version 1.15 this is the default. Note that this may
mean that ext2 filesystems created by a recent mke2fs cannot be
mounted r/w under Linux 2.0.*.) The block number here uses 1 k
units. Thus, if you want to use logical block 32768 on a
filesystem with 4 k blocks, use "sb=131072".
Support "user." extended attributes (or not).
Mount options for ext3
The ext3 filesystem is a version of the ext2 filesystem which has been
enhanced with journaling. It supports the same options as ext2 as well
as the following additions:
Update the ext3 filesystem's journal to the current format.
Don't load the journal on mounting. Note that if the filesystem
was not unmounted cleanly, skipping the journal replay will lead
to the filesystem containing inconsistencies that can lead to
any number of problems.
Specifies the journaling mode for file data. Metadata is always
journaled. To use modes other than ordered on the root filesys-
tem, pass the mode to the kernel as boot parameter, e.g. root-
All data is committed into the journal prior to being
written into the main filesystem.
This is the default mode. All data is forced directly
out to the main file system prior to its metadata being
committed to the journal.
Data ordering is not preserved - data may be written into
the main filesystem after its metadata has been committed
to the journal. This is rumoured to be the highest-
throughput option. It guarantees internal filesystem
integrity, however it can allow old data to appear in
files after a crash and journal recovery.
Just print an error message if an error occurs in a file data
buffer in ordered mode.
Abort the journal if an error occurs in a file data buffer in
barrier=0 / barrier=1
This disables / enables the use of write barriers in the jbd
code. barrier=0 disables, barrier=1 enables (default). This
also requires an IO stack which can support barriers, and if jbd
gets an error on a barrier write, it will disable barriers again
with a warning. Write barriers enforce proper on-disk ordering
of journal commits, making volatile disk write caches safe to
use, at some performance penalty. If your disks are battery-
backed in one way or another, disabling barriers may safely
Sync all data and metadata every nrsec seconds. The default
value is 5 seconds. Zero means default.
Mount options for ext4
The ext4 filesystem is an advanced level of the ext3 filesystem which
incorporates scalability and reliability enhancements for supporting
The options journal_dev, norecovery, noload, data, commit, orlov,
oldalloc, [no]user_xattr [no]acl, bsddf, minixdf, debug, errors,
data_err, grpid, bsdgroups, nogrpid sysvgroups, resgid, resuid, sb,
quota, noquota, grpquota, usrquota usrjquota, grpjquota and jqfmt are
backwardly compatible with ext3 or ext2.
Enable checksumming of the journal transactions. This will
allow the recovery code in e2fsck and the kernel to detect cor-
ruption in the kernel. It is a compatible change and will be
ignored by older kernels.
Commit block can be written to disk without waiting for descrip-
tor blocks. If enabled older kernels cannot mount the device.
This will enable 'journal_checksum' internally.
barrier=0 / barrier=1 / barrier / nobarrier
These mount options have the same effect as in ext3. The mount
options "barrier" and "nobarrier" are added for consistency with
other ext4 mount options.
The ext4 filesystem enables write barriers by default.
This tuning parameter controls the maximum number of inode table
blocks that ext4's inode table readahead algorithm will pre-read
into the buffer cache. The value must be a power of 2. The
default value is 32 blocks.
Number of filesystem blocks that mballoc will try to use for
allocation size and alignment. For RAID5/6 systems this should
be the number of data disks * RAID chunk size in filesystem
Deferring block allocation until write-out time.
Disable delayed allocation. Blocks are allocated when data is
copied from user to page cache.
Maximum amount of time ext4 should wait for additional filesys-
tem operations to be batch together with a synchronous write
operation. Since a synchronous write operation is going to force
a commit and then a wait for the I/O complete, it doesn't cost
This parameter sets the commit time (as described above) to be
at least min_batch_time. It defaults to zero microseconds.
Increasing this parameter may improve the throughput of multi-
threaded, synchronous workloads on very fast disks, at the cost
of increasing latency.
The I/O priority (from 0 to 7, where 0 is the highest priority)
which should be used for I/O operations submitted by kjournald2
during a commit operation. This defaults to 3, which is a
slightly higher priority than the default I/O priority.
abort Simulate the effects of calling ext4_abort() for debugging pur-
poses. This is normally used while remounting a filesystem
which is already mounted.
Many broken applications don't use fsync() when replacing exist-
ing files via patterns such as
fd = open("foo.new")/write(fd,...)/close(fd)/ rename("foo.new",
or worse yet
fd = open("foo", O_TRUNC)/write(fd,...)/close(fd).
If auto_da_alloc is enabled, ext4 will detect the replace-via-
rename and replace-via-truncate patterns and force that any
delayed allocation blocks are allocated such that at the next
journal commit, in the default data=ordered mode, the data
blocks of the new file are forced to disk before the rename()
operation is committed. This provides roughly the same level of
guarantees as ext3, and avoids the "zero-length" problem that
can happen when a system crashes before the delayed allocation
blocks are forced to disk.
Do not initialize any uninitialized inode table blocks in the
background. This feature may be used by installation CD's so
that the install process can complete as quickly as possible;
the inode table initialization process would then be deferred
until the next time the filesystem is mounted.
The lazy itable init code will wait n times the number of mil-
liseconds it took to zero out the previous block group's inode
table. This minimizes the impact on system performance while the
filesystem's inode table is being initialized.
Controls whether ext4 should issue discard/TRIM commands to the
to quickly locate extents which might overlap with filesystem
metadata blocks. This option is intended for debugging purposes
and since it negatively affects the performance, it is off by
Controls whether or not ext4 should use the DIO read locking. If
the dioread_nolock option is specified ext4 will allocate unini-
tialized extent before buffer write and convert the extent to
initialized after IO completes. This approach allows ext4 code
to avoid using inode mutex, which improves scalability on high
speed storages. However this does not work with data journaling
and dioread_nolock option will be ignored with kernel warning.
Note that dioread_nolock code path is only used for extent-based
files. Because of the restrictions this options comprises it is
off by default (e.g. dioread_lock).
This limits the size of the directories so that any attempt to
expand them beyond the specified limit in kilobytes will cause
an ENOSPC error. This is useful in memory-constrained environ-
ments, where a very large directory can cause severe performance
problems or even provoke the Out Of Memory killer. (For example,
if there is only 512 MB memory available, a 176 MB directory may
seriously cramp the system's style.)
Enable 64-bit inode version support. This option is off by
The ext2, ext3, and ext4 filesystems support setting the following file
attributes on Linux systems using the chattr(1) utility:
a - append only
A - no atime updates
d - no dump
D - synchronous directory updates
i - immutable
S - synchronous updates
u - undeletable
In addition, the ext3 and ext4 filesystems support the following flag:
j - data journaling
Finally, the ext4 filesystem also supports the following flag:
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